1. Field of the Invention
This invention relates to a rubber composition suitable for a rubber component for tire.
2. Prior Art
As a rubber composition for tire tread, there has been used a rubber composition comprising as the main component a styrene-butadiene copolymer having a relatively high glass transition temperature because, as for a tire of an automobile travelling on a general road, great importance is attached to braking performance on a wet road surface (wet skid performance) and controllability on a wet road surface. On the other hand, as for a tire for use in wintertime, great importance is attached to braking performance on ice and snow (ice skid performance), and hence, a polybutadiene rubber having a low glass transition temperature or natural rubber has been used.
With respect to the wet skid performance, it is important to increase the frictional force against road surface by increasing the hysteresis loss of tread rubber composition. That is to say, the tread surface which is being rubbed with road surface undergoes a high speed deformation by the fine irregularities of the road surface, and the larger the energy dissipation caused by the hysteresis loss produced during the periodic deformation course, the larger the frictional force. Since the deformation at the frictional interface takes place at a very high speed, it can be assumed based on the Williams-Landel-Ferry time-temperature superposition principle that the frictional force depends upon the hysteresis loss measured at a lower temperature than the tire-use temperature. Particularly, the friction coefficient of tire and tan .delta. (loss tangent), which is the measure of hysteresis loss, measured at around 0.degree. C. show a good correlation.
On the other hand, as to the ice skid performance, it is important to increase the frictional coefficient against road surface by allowing a tread rubber composition to have a high flexibility at a low temperature (-20.degree. C. or thereabout). At such low temperatures, the modulus of elasticity of a rubber composition increases, whereby it becomes difficult for the tread to follow the irregularities of road surface, and a frozen road surface is less uneven than a usual road surface. And hence, the deformation of tread surface decreases and the contribution of the energy dissipation (tan .delta.) produced at the interface between rubber and road surface to the ice skid performance becomes small. It is necessary to increase the real contact area between the tread surface and the road surface at low temperatures and it becomes more important to lower the storage modulus E' at about -20.degree. C. (making the modulus of elasticity low).
There has been demanded a rubber composition for tire tread which has a further improved wet skid performance and ice skid performance, and, at the same time, has a performance capable of sufficiently satisfying the abrasion resistance.
JP-A-62-179,542 discloses a polybutadiene rubber composition suitable for the rubber component of a high-impact polystyrene resin, a composition which consists of 70 to 30% by weight of a high molecular weight polybutadiene comprising, as the main component, a polybutadiene having an intrinsic viscosity [.eta.] of 3.0 to 7.0 and a cis-1,4-configuration, and 30 to 70% by weight of a low molecular weight polybutadiene comprising, as the main component, a polybutadiene having an intrinsic viscosity [.eta.] of 0.5 to 1.4 and a cis-1,4-configuration. However, it has no statement of any application thereof to tire.
Moreover, JP-A-4-100,810 discloses a polybutadiene rubber composition suitable for the rubber component of a high-impact polystyrene resin, a composition which is composed of 80 to 30% by weight of a high molecular-weight polybutadiene, comprising as the main component, a polybutadiene having an intrinsic viscosity [.eta.] of 3.0 to 7.0 and a cis-1,4-configuration content of at least 80%, and 20 to 70% by weight of a low molecular weight polybutadiene comprising, as the main component, a polybutadiene having an intrinsic viscosity [.eta.] of 0.5 to 1.4 and a cis-1,4-configuration content of less than 80%. However, it has no statement of any application thereof to a tire.
Furthermore, JP-B-42-9,017 discloses a rubber composition consisting of 70 to 95% by weight of a high molecular weight polybutadiene having an intrinsic viscosity [.eta.] of 1.5 to 20 and a cis-1,4-configuration content of at least 85%, and 30 to 5% by weight of a low molecular weight polybutadiene having an intrinsic viscosity [.eta.] of 0.35 to 0.75.